1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus having such a sheet feeding apparatus, and more particularly, it relates to a sheet feeding apparatus in which sheets are separated by using a separating inclined surface.
2. Related Background Art
In conventional image forming apparatuses such as printers, copying machines, facsimile apparatuses and the like, there is provided a sheet feeding apparatus for feeding a sheet to an image forming portion. Among such sheet feeding apparatuses, there is a sheet feeding apparatus in which sheets are separated one by one by a separating inclined surface to be supplied to the image forming portion.
FIG. 41 is a schematic constructural view of an image forming apparatus having such a conventional sheet feeding apparatus. In FIG. 41, the image forming apparatus 100 includes a sheet feeding tray 101 having a horizontal sheet stacking surface 101a, and a separating inclined surface 102 disposed at an end of the sheet feeding tray 101 in a sheet feeding direction. The separating inclined surface 102 is inclined with respect to a sheet (stack) S by an angle of xcex8 (obtuse angle). Incidentally, there is further provided a trailing end regulating member 120 for supporting a trailing end of the sheet (stack) S rested on the sheet stacking surface, and a position of such a member can be adjusted.
The sheet feeding apparatus includes a sheet feeding roller 103 to which a gear 103a is integrally formed, a sheet feeding roller arm 105 adapted to rotatably support the sheet feeding roller 103 and rotatably held on a driving shaft 104, a driving gear 106 secured to the driving shaft 104, and idler gears 107a to 107d held on the sheet feeding roller arm 105 and serving to transmit a driving force of the driving gear 106 to the sheet feeding roller 103. Incidentally, the driving shaft 104 is driven by a controllable driving mechanism (not shown) so that start and stop of an sheet feeding operation can be controlled by the driving of the driving shaft 104.
There are further provided a conveying sub-roller 109 urged against a conveying roller 108 with predetermined pressure by means of biasing means (not shown) (the sheet S fed out by the sheet feeding roller 103 is conveyed by rotations of the conveying roller 108 and the conveying sub-roller 109), an upper surface conveying guide 111 for guiding an upper surface of the sheet, and a lower surface conveying guide 110 for guiding a lower surface of the sheet. The sheet feeding tray 101, sheet feeding roller 103, sheet feeding roller arm 105 and the like constitute the sheet feeding apparatus 100A.
On the other hand, in FIG. 41, there are further provided a photosensitive drum 112, a toner cartridge 113 for integrally holding the photosensitive drum 112 and image developing means (not shown), a laser scanner 114 for forming a latent image on the photosensitive drum by exposing the surface of the photosensitive drum, and a transfer roller 115 for transferring a toner image (formed on the photosensitive drum by developing the latent image formed on the photosensitive drum in the toner cartridge) onto the sheet S. The photosensitive drum 112 and the transfer roller 115 are parts of an image forming portion 100B.
There are further provided a fixing device 116 for fixing the toner image transferred by the transfer roller 115 onto the sheet S, a sheet discharging roller pair 117 for conveying the sheet S to which the toner image was fixed, a sheet discharging roller pair 118 for discharging the sheet S out of the image forming apparatus, and a sheet discharge tray 119 on which the sheet S is discharged.
When image formation is effected in the image forming apparatus 100 having the above mentioned construction, first of all, the latent image is formed on the surface of the photosensitive drum 112 by the exposure effected by the laser scanner 114, and, thereafter, the toner image is formed on the surface of the photosensitive drum by developing the latent image by means of the image developing means provided in the toner cartridge 113.
On the other hand, in synchronous with the above operation, the driving shaft 104 is rotated by the driving mechanism (not shown), and the rotation of the driving shaft is transmitted to the gear 103a of the sheet feeding roller 103 via the idler gears 107a to 107d born on the sheet feeding roller arm 105, thereby starting to rotate the sheet feeding roller 103.
By the way, as shown in FIG. 42, since the sheet feeding roller 103 is urged against an upper surface of an uppermost sheet S1 included in the sheet stack S (referred to as xe2x80x9cuppermost sheetxe2x80x9d hereinafter) rested on the sheet stacking surface, when the rotation of the sheet feeding roller 103 is started in this way, a feeding force Fp due to a friction force acts on the uppermost sheet S1. Further, at the same time, the uppermost sheet S1 is subjected to a friction force Fs12 as a resistance force from an underlying next sheet S2.
As a result, the uppermost sheet S1 abuts against the separating inclined surface 102 and is subjected to a reaction force R1 from the separating inclined surface 102, with the result that a leading end portion of the uppermost sheet is bent upwardly. Thereafter, when the sheet feeding roller 103 is further rotated, the bent sheet S1 is further advanced while abutting the leading end thereof against the separating inclined surface. Incidentally, since the next sheet S2 is subjected to a friction force Fs23 from an underlying sheet S3, the next sheet does not advance together with the uppermost sheet S1, thereby preventing double-feeding.
The uppermost sheet S1 separated and fed by the sheet feeding roller 103 and the separating inclined surface 102 in this way is conveyed by the rotations of the conveying roller 108 and the conveying sub-roller 109 shown in FIG. 41 and is sent to a nip between the photosensitive drum 112 and the transfer roller 115.
Thereafter, the toner image formed on the photosensitive drum is transferred onto the sheet S1 (sent to the nip between the photosensitive drum 112 and the transfer roller 115) by the transfer roller 115, and the sheet S1 to which the toner image was transferred in this way is conveyed to the fixing device 116. After the toner image was fixed in the fixing device 116, the sheet S1 is discharged out of a main body 100C of the image forming apparatus and is rested on the sheet discharge tray 119.
By the way, in such a conventional sheet feeding apparatus and image forming apparatus having the sheet feeding apparatus, as mentioned above, the prevention of the double-feeding of sheets relies upon deformation resistance of the sheet and the friction force between the sheets.
Thus, for example, when new sheets are replenished before the previously replenished sheets are used up, if coefficient of friction in the interface between the newly replenished sheet and the previously existing sheet is lower than coefficient of friction before other sheets, the sheet separation will be effected only by the deformation resistance of the sheet on the separating inclined surface 102, with the result that, regarding sheets having low resiliency such as thin sheets, double-feeding may occur frequently in such an interface.
In order to prevent the double-feeding of thin sheets efficiently, for example, although there has been proposed a technique in which a distance between the sheet feeding roller 103 and the separating inclined surface 102 is decreased, if such a technique is used, the double-feeding of thin sheets can be prevented, but, a force required for feeding a thick sheet will become insufficient, thereby causing poor sheet feeding.
The present invention is made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide a sheet feeding apparatus in which double feeding of thin sheets can be prevented positively and a thick sheet can be fed positively.
To achieve the above object, the present invention provides a sheet feeding apparatus comprising sheet feeding means abutting against a sheet contained in sheet containing means to feed out the sheet, and a separating inclined surface for separating and feeding the sheet fed out by the sheet feeding means one by one, wherein the separating inclined surface can be shifted along a sheet advancing direction with the sheet fed out by the sheet feeding means.